This invention relates to a phase detector which monitors the presence, sequence and polarity of each phase of a three-phase line power input and provides an immediate indication when one or more of the phases is missing, out of sequence or of incorrect polarity.
Three-phase, alternating current, line powr inuts are commonly used to operate electric motors. The frequencies and voltages of three-phase inputs vary widely. Control of a motor driven by a three-phase line input can be lost if one or more phase line inputs is lost due to a malfunction, for example such as an overload condition, which causes safety devices in one or more phase lines to disconnect these lines. Control of a motor also can be lost if the three-phase lines are connected improperly and the phase lines are not of sequence or of incorrect polarity. Polarity of a three-phase line input is important because in some motors power is supplied to the motor during the positive portion of the line phases to cause the motor to rotate in one direction and during the negative portion of the line phases to cause the motor to rotate in the opposite direction.
Because a missing phase, an out of sequence phase or a phase of the wrong polarity can disrupt the operation of a device such as an electric motor, which is driven by the three-phase line power input, it is desirable to have a device which can be connected to a three-phase line power input and monitor the presence, the sequence and polarity of the phases in each line. In the event a phase is lost, out of sequence, or of the wrong polarity, the device should provide an immediate indication of such condition.
Presently existing phase detectors, which monitor three-phase line power inputs, can function only over a very narrow range of line power input voltages. Any change in input voltage requires numerous adjustments of the detector. Three-phase line power inputs commonly range from 120 volts to approximately 600 volts. However, a phase detector which is used to monitor the three-phase line power input of 120 volts cannot be used to monitor a power input of 600 volts. The voltage range from 120 to 600 volts is too great. Consequently, different phase detector units must be used for three-phase line power inputs of substantially different voltages.
Existing phase detectors also are frequency sensitive. Consequently, a phase detector which is used with a three-phase line power input having a frequency of 60 Hz cannot be used to monitor a power input with a frequency of 50 Hz. Additionally, currently available phase detectors require the presence of at least two phase line inputs to operate. Therefore, they cannot provide an indication which differentiates between when a three-phase power input is completely dead and there is no power in any phase and when it is not completely dead and there is power in one phase line. The ability of a phase detector to distinguish between these two conditions would be extremely helpful to people who have to services devices connected to the three-phase lines.